Accompanying the diversification in services provided by means of radio communications, a capability for multiband operations whereby information is processed in a plurality of frequency bands is in demand of radio equipment. As an indispensable device included in a piece of radio equipment, there is the power amplifier. In order to carry out highly efficient amplification, there is a need to obtain impedance matching between the amplification element which actually amplifies the signal and the peripheral circuits thereof. For this use, a matching circuit is used. Further, the input/output impedance of a peripheral circuit is generally made to coincide at a certain fixed value Z0 (being e.g. 50Ω, 75Ω, or the like), and hereinafter, the input/output impedance of the peripheral circuit is chosen to be called “the system impedance”.
In FIG. 1, there is shown an example of the input and output scattering parameters (S-parameters) of an amplification element used in an amplifier. In this example, S11 is the input reflection coefficient of the amplification element in the case where the output load is 50Ω and S22 is the output reflection coefficient of the amplification element in the case where the input load is 50Ω. Also, by using these reflection coefficients and a Smith chart, it is possible to obtain the input/output impedance of the amplification element. The input/output impedance of the amplification element has, frequency-dependent characteristics, as shown with bold solid lines in FIG. 1. The values of the input/output impedance can be obtained from the S-parameters and the system impedance Z0. Consequently, in case an amplifier is designed using an amplification element like the one above, impedance matching between the input/output impedance of the amplification element and the system impedance Z0 at the design frequency is necessary. In other words, in the case of designing a multiband amplification circuit, impedance matching between the input/output impedance of the amplification element and the system impedance Z0 becomes necessary at a plurality of design frequencies.
Accordingly, in the case of amplifying signals in different frequency bands, there are, as in the amplifier used inside the band-sharing mobile terminal described in Reference 1 (Kouji Chiba et al., “Mobile Terminals”, NTT DoCoMo Technical Journal, Vol. 10, No. 1, pp. 15-20), (1) the method of providing amplifiers combining an amplification element and a matching circuit, wherein the number of the amplifiers equals to the number of frequency bands, and selecting an amplifier in response to the used frequency band, and (2) the method of providing one amplification element, which has an amplifiable frequency domain which is sufficiently wide with respect to the operating frequency band of the matching circuit, and a matching circuit capable of modifying each parameters of the circuit elements in response to the frequency band in which an amplifier operates. In particular, method (2) has the advantage that a reduction in the size of the amplifier is possible in comparison with method (1).
In FIG. 2, there is shown an example of a multiband matching circuit having small losses, shown in Reference 2 (A. Fukuda, H. Okazaki, T. Hirota, and Y. Yamao, “Multi-band Power Amplifier Employing MEMS Switches for Optimum Matching”, C-2-4, 2004). A multiband matching circuit 900 comprises a main matching block 910, a delay circuit 920 connected at one end thereof to main matching block 910, an auxiliary matching block 930, and a switch element 940 connected between the other end of delay circuit 920 and one end of auxiliary matching block 930. In the case where a load 1020 having impedance frequency characteristics ZL(f) connects to a port 952, multiband matching circuit 900 is a matching circuit which matches impedances between the impedance, seen from a port 951 toward the side of multiband matching circuit 900, and the impedance Z0 of a load 1010 in the signal band. E.g., it is possible, as shown in FIG. 3, to match the impedances in two frequency bands; one has center frequency f1, and the other has center frequency f2.
First, an explanation will be given regarding impedance matching at frequency f1. In this case, switch element 940 is chosen to be in the OFF state. A signal input from e.g. the side of port 952 passes through main matching block 910 and delay circuit 920, and is transmitted to the side of port 951. Here, main matching block 910 can be composed of one or more arbitrary circuitry and is chosen to be the circuit which matches between impedance ZL(f1) and impedance Z0 at frequency f1. Also, delay circuit 920 is chosen to be the transmission line having a characteristic impedance Z0. Consequently, the value of the impedance seen from a connection point 953, shown in FIG. 2, between main matching block 910 and delay circuit 920, toward the side of port 951 is Z0. In other words, multiband matching circuit 900 implements impedance matching with itself at frequency f1.
Next, assuming the aforementioned design of impedance matching at frequency f1, an explanation will be given regarding impedance matching at frequency f2. In this case, switch element 940 is chosen to be in the ON state. Main matching block 910 operates as an impedance converter at frequency f2. So, the value of the impedance seen from connection point 953 toward the side of port 952 is ZL′(f2), which is the result that the impedance was converted into by main matching block 910.
Without regard for the value of ZL′(f2), by appropriately setting the line length of delay circuit 920 which is a transmission line and the reactance value of auxiliary matching block 930 connected in parallel with delay circuit 920 as design items in advance, it is possible, on the basis of the single stub matching scheme, to convert the value of the impedance seen from port 951 toward the side of multiband matching circuit 900 to the value Z0. In short, multiband matching circuit 900 can obtain impedance matching with itself at frequency f2 as well.
By adding delay circuit 920 having a characteristic impedance Z0 and auxiliary matching block 930 connected to main matching block 910 via switch element 940, multiband matching circuit 900 can operate as a matching circuit both at frequency f1, and frequency f2. In short, multiband matching circuit 900 functions as a matching circuit for two frequency bands by switching the ON/OFF state of one switch element.
In recent years, there have been developed some kind of amplification elements, e.g. transistors, which have a high gain over a large bandwidth. Generally, the gain of an amplification elements increases as the frequency becomes lower and decreases as the frequency becomes higher. E.g., microwave band transistors and the like, capable of amplification in frequency bands as high as several gigahertz have an exceedingly high amplification gain in low frequency bands at or below several tens of megahertz.
Commonly, some kind of feedback loop is formed in the periphery of the amplification element. In this case, if the gain of the same feedback loop exceeds 1, an oscillating condition is satisfied. So there is a possibility that parasitic oscillations occur. In order to prevent parasitic oscillations, no matter which load is connected to the amplification element, it is important that, across the entire frequency band, oscillating condition is not satisfied (i.e., that stable condition is satisfied). Accordingly, stabilization circuits are used in amplifiers.
A stabilization circuit is normally designed so that no influence is exerted in high frequency bands in which amplification is carried out and so that the gain of the feedback loop is lowered at low frequency bands in which parasitic oscillations occur easily. And then, the stabilization circuit is connected to both the input and output terminals of an amplification element, or in series on one side of, or in parallel with, the amplification element. By means of a stabilization circuit connected close to an amplification element, the amplification element operates stably. Consequently, by means of the stabilization circuit, the amplifier can obtain a necessary gain in the high frequency bands in which amplification is carried out and the parasitic oscillations do not occur in the low frequency bands.
In FIG. 4, there is shown an example of a conventional stabilization circuit. Stabilization circuit 960 comprises a resistor 961 and a capacitor 962 connected in parallel with the resistor 961. Stabilization circuit 960 is connected to an amplification element 970 exemplified by a transistor, a FET (Field Effect Transistor), a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), a TFT (Thin Film Transistor), or the like, after appropriately setting, as design items, the design values of the resistor 961 and the capacitor 962. For circuit comprising amplification element 970 and stabilization circuit 960, oscillation condition is not satisfied in any part of the frequency band. In other words, this circuit is a stabilized circuit. Hereinafter, this circuit is also called “a stabilized amplification element”.